As an optical disc recording medium, CD (compact disc) and MD (mini-disc) (these are registered as trademarks) are widely spreading and used in various application fields such as audio, etc.
In a disc reproducing apparatus for reproducing various types of recording media, in order to control the tracking of an optical spot, there are provided a double-axis mechanism for driving an objective lens of an optical head with a tracking error signal obtained from a reflected light beam indicating a track guide information such as pit train and group, etc. and a sled mechanism for displacing the relative position of the optical head as a whole and disc surface in the disc radius direction.
As the sled mechanism, those for moving the optical head as a whole against a disc and for moving a turn-table on which a disc is mounted against the optical head which is fixed to the predetermined position are known. Moreover, as a control system of the sled mechanism, it has been proposed that a sled error signal generated by extraction of a low frequency element with a low-pass filter from the tracking error signal is amplified and is then applied to a sled motor as a sled drive signal. The sled error signal is changed to the signal indicating an offset of an objective lens driven for tracking by the double-axis mechanism of the optical head as a whole and the optical head.
Here, FIG. 1 schematically shows an example of the structure of an optical block of the related art which is installed in an optical head of a reproducing apparatus for a disc such as CD, etc.
In this figure, an optical block of the optical head is composed of a semiconductor laser 81, a collimator lens 82, a deflected beam splitter 83, an objective lens 84 and a photo detector 85. For example, the laser beam radiated from the semiconductor laser 81 is paralleled by the collimator lens 82, and then reflected by the deflected beam splitter 83 to radiate the recording surface of a disc 1 via the objective lens 84.
When the laser beam is focused, the light beam reflected by the pit train provided on the disc 1 passes through the deflected beam splitter 83 and is then supplied to the photo detector 85 to provide the pit information of the disc 1 in the photo detector 85.
FIG. 2 shows the concept of distribution of intensity of the reflected light beam in the relative positional relationship between the pits formed on the disc 1 and spot beam. In the so-called just tracking condition where the pit train of disc 1 is relatively matched with the position of the spot beam, the reflected beam indicated as (a) can be obtained on the photo detector 84. Namely, pit information having an equal intensity distribution in the right and left sides can be obtained on the photo detector 85.
Moreover, if the pit of the disc 1 is relatively displaced in position from the spot beam, for example, when the spot beam position is relatively deviated in the left side from the pit train (de-track condition), the pit information having intensity distribution as shown in (b) can be obtained on the photo detector 85, while when the spot beam position is relatively deviated in the right side from the pit train (de-track condition), the pit information having intensity distribution as shown in (c) can be obtained on the photo detector 85.
Namely, when positions of the pit provided on the disc 1 and spot beam are relatively deviated in the tracking direction, the pit information having different intensity distribution in the right and left sides can be obtained on the photo detector 85.
A difference voltage obtained from intensity distribution difference in the right and left sides of the pit information obtained on the photo detector 85 is supplied to the double-axis mechanism to drive the objective lens 84 as a tracking error signal, a low frequency element of this tracking error signal is extracted to generate a sled error signal and it is then supplied to the sled mechanism for moving the optical head as a whole in order to control the tracking of the optical head to the ON-track condition. A method of detecting such tracking error signal is generally called a push-pull method and a tracking error signal obtained by this push-pull method is called a push-pull error signal in this specification of the present invention.
When the tracking of the reproducing apparatus is controlled by the push-pull method shown in FIG. 1, if the objective lens 84 of the optical head is shifted in the lateral direction (tracking direction) indicated by a broken line in FIG. 1 due to the tracking servo operation, etc., the reflected beam obtained on the photo detector 85 is also shifted in the tracking direction as indicated by the broken line.
Therefore, the push-pull error signal generated from the pit information obtained on the photo detector 85 includes, even under the just tracking condition, a DC offset voltage depending on the shift of the objective lens 84, resulting in the problem that this DC offset voltage disables accurate tracking control.
In view of solving such a problem, a detecting method called the top hold push-pull method has been proposed. In this method, the tracking error signal is detected after removing the DC offset voltage for detecting the tracking error signal. Such Top Hold Push-Pull (TPP) method has been disclosed in the Japanese Laid-Open Patent Number HEI 4-23234 presented (filed May 18, 1990) by the same applicant of the present invention. In this case, a DC offset removing circuit for removing a DC offset voltage included in the push-pull error signal is provided to obtain the tracking error signal after having removed the DC offset voltage. Thereby, accurate tracking control can be realized by eliminating influence of the shift operation in the tracking direction of the objective lens 84 of the optical head. In the related art, the sled error signal has been generated by extracting a low frequency element of the tracking error signal obtained by the top hole push-pull method explained above.
In general, since a reproducing apparatus for reproducing a disc such as CD or MD is restricted in installation space, it is expected that the apparatus may be installed vertically or horizontally and is reduced in size. The reproducing apparatus is required, even if it is installed vertically or horizontally, that the optical block in the optical head (at least the objective lens) may be held reliably and an arm forming the double-axis mechanism is never shifted in the tracking direction (horizontal direction) due to its gravity. Namely, if the reproducing apparatus is installed, for example, vertically, it is just preferable as shown in FIG. 3A that the disc 1 is arranged in vertical, the sled axis 91 of the optical head 90 is arranged in vertical and the arm 85 holding the objective lens 84 of the optical head 90 is arranged to cross diagonally the sled axis 91 so that the position in the tracking direction of the objective lens 84 is never influenced by gravity.
However, in this case, since the sled mechanism for moving the optical head 90 in the radius direction of the disc 1 becomes larger than the external shape of the disc 1 as indicated as the shaded area of FIG. 3A, it has impeded reduction in size of the reproducing apparatus.
Therefore, when the sled mechanism is inclined for arrangement, for example, as shown in FIG. 3B, the sled mechanism can be accommodated in the internal side as much as .DELTA.1 in comparison with FIG. 3A to realize reduction in size of the reproducing apparatus. However, since the sled axis 91 holding the objective lens 84 is also inclined, the arm 85 is deviated with its self weight and the objective lens 84 is shifted in the tracking direction (horizontal direction) under the natural condition where the tracking servo is not effectuated.
In the case where the tracking error signal is obtained by the top push-pull method in the reproducing apparatus in which the sled mechanism is inclined as shown in FIG. 3B, the DC offset voltage indicating the amount of shift due to the gravity of the objective lens 84 can be eliminated. Therefore, as schematically shown in FIG. 4B, the arm 85 holding the objective lens 84 does not return up to the actual mechanical center (hereinafter referred to as mechacenter) and thereby the arm 85 is shifted by its gravity to the external or internal circumference side of the disc 1.
When the optical block 84 as a whole of the optical head 90 is shifted to the external or internal circumference side of the disc 1 as explained above, in regard to the visual field indicated by the peak to peak level of the reproduced RF signal as shown in FIG. 11A and jitter element, the reproduced RF signal will be deteriorated because the peak to peak level of the reproduced RF signal is reduced and jitter element is increased depending on the movement of lens. As a result, when a disc is reproduced using a disc reproducing apparatus, there has been the problems that the vibration proof characteristic which is indicated by an applied vibration frequency showing the limit for not generating track skip may be deteriorated or the S/N ratio of the RF signal is deteriorated to generate noise.